@chris-a

Hi Chris,

Welcome to the forum.

I would be interested with more details about your solar thermal system. Does it provide hot water throughout the whole year or just from Spring through to Autumn?

Posted by: @derek-m

I would be interested with more details about your solar thermal system

Me too @chris-a

@derek-m and I have had a previous discussion about whether solar thermal should continue to be used as an input to a DHW cylinder, of if it's better to re-configure it as a pre-heater to a heat-pump when one gets installed. We don't (yet) have any answer on this, so feel free to chip in!

@transparent and Derek M,

Thanks. We've had our solar thermal system for 9 years of trouble-tree (and nearly maintenance free) operation: it's a very simple and efficient system provided the sun is shining (unlike PV, it prefers direct sunlight rather than just lux). Whilst we had our oil boiler it made a lot of sense, as it gave us free hot water for most of summer, with a top up from oil on cloudy days.

To answer Derek M's question, it is mostly (though not exclusively) Spring to autumn. It actually works well on sunny winter days: the outside temperature doesn't matter. On a day like this (sunny with clouds) it has taken most of the 300 litre cylinder up to 64 degrees C (sensor at the bottom, as is the solar coil). Even on cloudy but bright days, it raises the bottom of the tank to around 40 degrees. o even if it's not doing the whole job, it's giving the heat pump/boiler less work to do.

Transparent - that's an interesting question. We're not set up for it to input to the heating circuit, so for us it will remain for hot water. The advantage to that is that it can raise the DHW temperature far hotter than would be efficient for the heat pump (we have a thermostatic mixing valve to prevent scalds, so a hotter tank just means more hot water). How the heat pump would react to having the heating circuit raised to above its flow design temperature I'm not sure! When the conditions are right, it can dump a LOT of heat into the cylinder. Sadly for heating, during winter when you need it most, it will do less. But of course one needs DHW all year round, so perhaps that's another argument in its favour.

Another question might be, does it even make sense to have solar thermal when PV diverters can be used for the same thing, and the PV also powers the heat pump? This wasn't an option when we put it in, but I wonder if it might be a better use of roof space for heat pump owners. But I think Solar thermal is more efficient, provided the sun is shining.

@chris-a

Thanks for the info. My research indicates that solar thermal is up to 4 times more efficient than solar PV, so you could get the same amount of energy from 1/4 the roof space. We have solar PV, which provides the vast majority of our hot water from Spring through to Autumn and also assist the gas boiler in producing hot water in the Winter. So very similar to your solar thermal, but with the added advantage that it has reduced our electrical consumption by 50% over a full year.

Since you are having a quite large 18kW heat pump installed, solar PV would be somewhat beneficial, particular in the Spring/Autumn period, but will not be able to fully power your heat pump during the heating season. But as they say, 'every little helps'.

Posted by: @chris-a

How the heat pump would react to having the heating circuit raised to above its flow design temperature I'm not sure!

Precisely!

And for this reason I've just copied your text and the response from @derek-m into a new topic in the area about Solar Thermal. I think that's where others would expect to find it in months to come.

I've also added a couple of diagrams to make it easier to see what we're discussing. Feel free to re-edit those as you wish!